Project Summary CD4+ T follicular helper (Tfh) cells are essential for germinal center (GC) responses and long- term humoral immunity. However, the complex regulation that determines the differentiation of Tfh cells, their developmental progression in GC, and their number and function in helping GC B cell differentiation and antibody affinity selection, are still not fully understood. Our long-term goals are to identify novel pathways underlying the differentiation of Tfh cells and design new strategies to manipulate humoral responses for treatment of infectious diseases and autoimmune disorders and to aid vaccine development. Recently we have identified transcription factor Foxp1 as a rate-limiting and critical negative regulator of Tfh cell differentiation, which drastically affects both the kinetics and the magnitude of the antibody responses. Our new preliminary data now start to reveal that Foxp1 engages a complex network of regulators that are coordinated to suppress Tfh cell differentiation as well as Tfh help to B cells at different stages/steps of the GC response. In this application, we aim to identify Foxp1 targets and elucidate the molecular mechanisms by which Foxp1 regulates Tfh cell differentiation. In addition, we will further delineate how Foxp1-mediated regulation of Tfh cell differentiation affects GC B cell development, antibody affinity selection and chronic infection. Our study will not only reveal new components of the transcriptional regulation in Tfh cell differentiation, but also provide fundamental mechanistic insights in the regulation of GC B cell responses and antibody affinity selection.